Search Results (173 total)

We present a new determination of the parity of the neutral pion via the double Dalitz decay π⁰→e⁺e^-e⁺e^-. Our sample, which consists of 30 511 candidate decays, was collected from K_L→π⁰π⁰π⁰ decays in flight at the KTeV-E799 experiment at Fermi National Accelerator Laboratory. We confirm the negative π⁰ parity and place a limit on scalar contributions to the π⁰→e⁺e^-e⁺e^- decay amplitude of less than 3.3% assuming CPT conservation. The π⁰γ^*γ^* form factor is well described by a momentum-dependent model with a slope parameter fit to the final state phase-space distribution. Additionally, we have measured the branching ratio of this mode to be B(π⁰→e⁺e^-e⁺e^-)=(3.26±0.18)×10^-5.

The Fermilab KTeV experiment has searched for lepton-flavor-violating decays of the K_L meson in three decay modes. We observe no events in the signal region for any of the modes studied, and we set the following upper limits for their branching ratios at the 90% C.L.: BR(K_L→π⁰μ^±e^∓)<7.6×10^-11; BR(K_L→π⁰π⁰μ^±e^∓)<1.7×10^-10; BR(π⁰→μ^±e^∓)<3.6×10^-10. This result represents a factor of 82 improvement in the branching ratio limit for K_L→π⁰μ^±e^∓ and is the first reported limit for K_L→π⁰π⁰μ^±e^∓.

The cross section for high-E_T dijet production in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 81.8  pb^-1. The events were required to have a virtuality of the incoming photon, Q², of less than 1  GeV² and a photon-proton center-of-mass energy in the range 142<W_γp<293  GeV. Events were selected if at least two jets satisfied the transverse-energy requirements of E_T^jet1>20  GeV and E_T^jet2>15  GeV and pseudorapidity (with respect to the proton beam direction) requirements of -1<η^jet1,2<3, with at least one of the jets satisfying -1<η^jet<2.5. The measurements show sensitivity to the parton distributions in the photon and proton and to effects beyond next-to-leading order in QCD. Hence these data can be used to constrain further the parton densities in the proton and photon.

We report on a new measurement of the branching ratio B(K_L→π⁰e⁺e^-γ) using the KTeV detector. This analysis uses the full KTeV data set collected from 1997 to 2000. We reconstruct 139 events over a background of 14, which results in B(K_L→π⁰e⁺e^-γ)=(1.62±0.14_stat±0.09_syst)×10^-8. This result supersedes the earlier KTeV measurement of this branching ratio.

This Letter is the first report of the KL→π±e∓νe+e- decay. Based on 19 208±144 events, we determine the branching fraction, B(KL→π±e∓νe+e-;Me+e->5  MeV/c2,Ee+e-*>30  MeV)=(1.285±0.041)×10-5, and Γ(Ke3ee;Me+e->5  MeV/c2)/Γ(Ke3)=[4.57±0.04(stat)±0.14(syst)]×10-5. This ratio agrees with a theoretical prediction based on chiral perturbation theory (ChPT) calculated to O(p4). The measured kinematical distributions agree with those predicted by just ChPT O(p4), but show significant disagreement with ones predicted by leading-order ChPT.

The E799-II (KTeV) experiment at Fermilab has collected 83 262 K_L→e⁺e^-γ(γ) events above a background of 79 events. We measure a decay width, normalized to the K_L→π⁰π⁰π_D⁰ (π⁰→γγ, π⁰→γγ, π_D⁰→e⁺e^-γ(γ)) decay width, of Γ(K_L→e⁺e^-γ(γ))/Γ(K_L→π⁰π⁰π_D⁰)=(1.3302±0.0046_stat±0.0102_syst)×10^-3. We also measure parameters of two K_Lγ^*γ form factor models. In the Bergström-Massó-Singer parametrization, we find Cα_K^*=-0.517±0.030_stat±0.022_syst. We separately fit for the first parameter of the D’Ambrosio-Isidori-Portolés model and find α_DIP=-1.729±0.043_stat±0.028_syst.

The polarization of Ξ⁰ and Ξ̅ ⁰ hyperons produced by 800  GeV/c protons on a BeO target at a fixed targeting angle of 4.8 mrad is measured by the KTeV experiment at Fermilab. Our result of 9.7% for Ξ⁰ polarization shows no significant energy dependence when compared to a result obtained at 400  GeV/c production energy and at twice our targeting angle. The polarization of the Ξ̅ ⁰ is measured for the first time and found to be consistent with zero. We also examine the dependence of polarization on production p_t.

The branching ratio of the rare decay π⁰→e⁺e^- has been measured precisely, using the complete data set from the KTeV E799-II experiment at Fermilab. We observe 794 candidate π⁰→e⁺e^- events using K_L→3π⁰ as a source of tagged π⁰s. The expected background is 52.7±11.2 events, predominantly from high e⁺e^- mass π⁰→e⁺e^-γ decays. We have measured B(π⁰→e⁺e^-,(m_e⁺e^-/m_π⁰)²>0.95)=(6.44±0.25_stat±0.22_syst)×10^-8, which is above the unitary bound from π⁰→γγ and within the range of theoretical expectations from the standard model.

Using the published KTeV sample of 2×10⁶ K_L→π^±e^∓ν decays and a new form factor expansion with a rigorous bound on higher order terms , we present a new determination of the K_L→π^±e^∓ν form factor and phase space integral. Compared to the previous KTeV result , the uncertainty in the new form factor expansion is negligible and results in an overall uncertainty in the phase space integral (I_K^e) that is a factor of 2 smaller: I_K^e=0.15392±0.00048.

In this paper the KTeV collaboration reports the analysis of 112.1×10³ candidate K_L→π⁺π^-γ decays including a background of 671±41 events with the objective of determining the photon production mechanisms intrinsic to the decay process. These decays have been analyzed to extract the relative contributions of the CP violating bremsstrahlung process and the CP conserving M1 and CP violating E1 direct photon emission processes. The M1 direct photon emission amplitude and its associated vector form factor parameterized as |g˜_M1|(1+a₁/a₂ / (M_ρ²-M_K²)+2M_KE_γ) have been measured to be |g˜_M1|=1.198±0.035(stat)±0.086(syst) and a₁/a₂=-0.738±0.007(stat)±0.018(syst)  GeV²/c² respectively. An upper limit for the CP violating E1 direct emission amplitude |g_E1|≤0.21 (90%CL) has been found. The overall ratio of direct photon emission (DE) to total photon emission including the bremsstrahlung process (IB) has been determined to be DE/(DE+IB)=0.689±0.021 for E_γ≥20  MeV.

Using the complete KTeV data set of 5241 candidate KL→π+π-e+e- decays (including an estimated background of 204±14 events), we have measured the coupling gCR=0.163±0.014(stat)±0.023(syst) of the CP conserving charge radius process and from it determined a K0 charge radius of ⟨rK02⟩=[-0.077±0.007(stat)±0.011(syst)]fm2. We have determined a first experimental upper limit of 0.04 (90% C.L.) for the ratio |gE1| / |gM1| of the couplings for the E1 and M1 direct photon emission processes. We also report the measurement of |gM1| including a vector form factor |g˜M1|(1+a1/a2 / (Mρ2-MK2)+2MKEγ*), where |g˜M1|=1.11±0.12(stat)±0.08(syst) and a1/a2=[-0.744±0.027(stat)±0.032(syst)]  GeV2/c2. Finally, a CP-violating asymmetry of [13.6±1.4(stat)±1.5(syst)]% in the CP and T odd angle ϕ between the decay planes of the e+e- and π+π- pairs in the KL center of mass is reported.

The Ξ⁰ muon semileptonic decay has been observed for the first time with nine identified events using the KTeV beam line and detector at Fermilab. The decay is normalized to the Ξ⁰ beta decay mode and yields a value for the ratio of decay rates Γ(Ξ⁰→Σ⁺μ^-ν̅ _μ)/Γ(Ξ⁰→Σ⁺e^-ν̅ _e) of [1.8_-0.5^+0.7(stat)±0.2(syst)]×10^-2. This is in agreement with the SU(3) flavor symmetric quark model.

We present results on the production of direct photons, π⁰, and η mesons on nuclear targets at large transverse momenta (p_T). The data are from 530 and 800  GeV/c proton beams and 515  GeV/c π^- beams incident upon copper and beryllium targets that span the kinematic range of 1.0<p_T≲10  GeV/c at central rapidities.

We present measurements of R_Kℓ3γ≡Γ(K_L→π^±ℓ^∓νγ;E_γ^⋆>10  MeV)/Γ(K_L→π^±ℓ^∓ν), where ℓ=μ or e, and E_γ^⋆ is the photon energy in the kaon rest frame. These measurements are based on K_L decays collected in 1997 by the KTeV (E832) experiment at Fermilab. With samples of 1385 K_L→π^±μ^∓νγ and 14221 K_L→π^±e^∓νγ candidates, we find R_Kμ3γ=(0.530±0.019)% and R_Ke3γ=(4.942±0.062)%. We also examine distributions of photon energy and lepton-photon angle.

Measurements of the production of high transverse momentum direct photons by a 515  GeV/c π^- beam and 530 and 800  GeV/c proton beams in interactions with beryllium and hydrogen targets are presented. The data span the kinematic ranges of 3.5<p_T<12  GeV/c in transverse momentum and 1.5 units in rapidity. The inclusive direct-photon cross sections are compared with next-to-leading-order perturbative QCD calculations and expectations based on a phenomenological parton-k_T model.

We present new measurements of K_L semileptonic form factors using data collected in 1997 by the KTeV (E832) experiment at Fermilab. The measurements are based on 1.9×10⁶ K_L→π^±e^∓ν and 1.5×10⁶ K_L→π^±μ^∓ν decays. For f₊(t), we measure both a linear and quadratic term: λ₊^′=(20.64±1.75)×10^-3 and λ₊^′′=(3.20±0.69)×10^-3. For f₀(t), we find λ₀=(13.72±1.31)×10^-3. These form factors are consistent with K^± form factors, suggesting that isospin symmetry breaking effects are small. We use our measured values of the form factors to evaluate the decay phase space integrals, I_K^e=0.15350±0.00105 and I_K^μ=0.10165±0.00080, where errors include uncertainties arising from the form factor parametrizations.

We present new measurements of the six largest branching fractions of the K_L using data collected in 1997 by the KTeV experiment (E832) at Fermilab. The results are B(K_L→π^±e^∓ν)=0.4067±0.0011, B(K_L→π^±μ^∓ν)=0.2701±0.0009, B(K_L→π⁺π^-π⁰)=0.1252±0.0007, B(K_L→π⁰π⁰π⁰)=0.1945±0.0018, B(K_L→π⁺π^-)=(1.975±0.012)×10^-3, and B(K_L→π⁰π⁰)=(0.865±0.010)×10^-3, where statistical and systematic errors have been summed in quadrature. We also determine the CP violation parameter |η_+-| to be (2.228±0.010)×10^-3. Several of these results are not in good agreement with averages of previous measurements.

We present a determination of the Cabibbo-Kobayashi-Maskawa parameter |V_us| based on new measurements of the six largest K_L branching fractions and semileptonic form factors by the KTeV (E832) experiment at Fermilab. We find |V_us|=0.2252±0.0008_KTeV±0.0021_ext, where the errors are from KTeV measurements and from external sources. We also use the measured branching fractions to determine the CP violation parameter |η_+-|=(2.228±0.005_KTeV±0.009_ext)×10^-3.

Cross sections for e⁺p neutral current deep inelastic scattering have been measured at a center-of-mass energy of sqrt[s]=318   GeV with the ZEUS detector at HERA using an integrated luminosity of 63.2   pb^-1. The double-differential cross section, d²σ/dxdQ², is presented for 200   GeV²<Q²<30 000   GeV² and for 0.005<x<0.65. The single-differential cross sections dσ/dQ², dσ/dx and dσ/dy are presented for Q²>200   GeV². The effect of Z-boson-exchange is seen in dσ/dx measured for Q²>10 000   GeV². The data presented here were combined with ZEUS e⁺p neutral current data taken at sqrt[s]=300   GeV and the structure function F₂^em was extracted. All results agree well with the predictions of the Standard Model.

The photoproduction of bottom quarks in events with two jets and a muon has been measured with the ZEUS detector at HERA using an integrated luminosity of 110 pb^-1. The fraction of jets containing b quarks was extracted from the transverse momentum distribution of the muon relative to the closest jet. Differential cross sections for bottom production as a function of the transverse momentum and pseudorapidity of the muon, of the associated jet and of x_γ^jets, the fraction of the photon’s momentum participating in the hard process, are compared with MC models and QCD predictions made at next-to-leading order. The latter give a good description of the data.

The KTeV/E799 experiment at Fermilab has searched for the rare kaon decay K_L→π⁰e⁺e^- . This mode is expected to have a significant CP violating component. The measurement of its branching ratio could support the standard model or could indicate the existence of new physics. This Letter reports new results from the 1999–2000 data set. One event is observed with an expected background at 0.99±0.35 events. We set a limit on the branching ratio of 3.5×10^-10 at the 90% confidence level. Combin­ing with the previous result based on the data set taken in 1997 yields the final KTeV result: BR(K_L→π⁰e⁺e^-)<2.8×10^-10 at 90% C.L.

We present results on the production of high transverse momentum π⁰ and η mesons in π^-p and π^-Be interactions at 515 GeV/c. The data span the kinematic ranges 1<p_T<11 GeV/c in transverse momentum and -0.75<~y_cm<~0.75 in rapidity. The inclusive π⁰ cross sections are compared with next-to-leading order QCD calculations and to expectations based on a phenomenological parton-k_T model.